Method and device for melting glass using an induction-heated crucible with cooled crust

ABSTRACT

The invention describes a method and an apparatus for the rapid melting of in particular high-purity, aggressive and/or high-melting glasses in a skull crucible, in which high-frequency energy is introduced into the contents of the crucible by means of a coil arrangement surrounding the skull crucible, in order to heat the melt, and the batch is laid and the molten glass discharged in the upper region of the crucible, and undissolved constituents of the batch are retained by means of a cooled bridge which is immersed in the melt. According to the invention, the glass is taken off above the coil arrangement ( 1.3 ) and is fed for further processing without flowing through the coil region. This has the advantage that simple, conventional components can be used to connect the melting unit to the further-processing stations, and the type of connection can be selected in such a way that it is possible to prevent the quality of the glass from being impaired by connection techniques which are less than optimum.

The invention relates to a method and an apparatus for the rapid meltingof in particular high-purity, aggressive and high-melting glasses in askull crucible, in which high-frequency energy is introduced into thecontents of the crucible by means of a coil arrangement surrounding theskull crucible, in order to heat the melt, and the batch is laid and themolten glass discharged in the upper region of the crucible, andundissolved constituents of the batch are retained by means of a cooledbridge which is at least partially immersed in the melt from above.

Nowadays, aggressive glasses which are required to have a high purityare melted discontinuously in platinum crucibles or continuously inplatinum tank furnaces. Both the melting tank furnace and the refiningtank furnace and the homogenization tank furnace consist of platinum.This melting technology is disadvantageous on account of the high costsof the precious metal and also the short service lives of platinumequipment of this type. In particular the melting region, where batchreactions take place, is exposed to considerable corrosion and istherefore often the limiting component of a platinum tank furnace interms of the service lives. On account of the heating technology via theplatinum wall and the stability of the platinum, the maximum throughputswhich can be achieved with equipment of this type are less than 1 t perday for a melting tank furnace or crucible volume of 90 l.

In addition to platinum melting units, it is also known to use skullcrucibles, which are formed from water-cooled, spaced-apart metal tubesand in which the melt is heated by means of induction coils whichsurround the crucible, by high-frequency energy being radiated in.Melting equipment of this type has the advantage that the water coolingmeans that a protective layer of material of the same composition of theglass is inevitably formed in the edge region of the crucible, so as tosurround the melt in the form of a crucible of the same composition ofthe glass, so as to provide protection against impurities.

PETROV, YU. B. ET AL.: “Continuous casting glass melting in a coldcrucible induction furnace”, XV INTERNATIONAL CONGRESS ON GLASS 1989,PROCEEDINGS, Vol. 3a, 1989, pages 72-77 has disclosed a crucible of theabovementioned type for melting high-purity glasses. In this crucible,the batch is supplied in the upper crucible region and the glass islikewise taken off in the upper crucible region. The batch and outletregion are separated from one another by a cooled bridge, which isimmersed deep in the melt, in order to retain undissolved constituentsof the batch. The melted glass is taken off at the upper edge of thecrucible via an overflow channel arranged inside the coil and dropsdownward in the form of a glass strand between the crucible wall and theinternal radius of the coil.

The document does not give any information as to how the glass strand iscollected and then fed for further processing. However, it is obviousthat in the arrangement described the possibilities for connecting themelting unit to the further-processing units are very limited. Moreover,with the known procedure it is likely that the quantity of glass in theglass strand will be subject to fluctuations over the course of time, sothat at best only a quasi-continuous procedure is possible. A furtherdrawback is that the drop height of the glass strand has to be verygreat, since the glass has to drop through at least the entire height ofthe coil before it can be collected in a channel or tank furnacearranged outside the coil. Consequently, it is likely that bubbles willbe introduced into the melt and that the quality with regard to cordswill deteriorate. Furthermore, cooling of the glass in the glass strandmay be problematic in the case of high-melting glasses. The problem maybe that the glass is not guided and therefore starts to splash.Furthermore, sparkovers may occur between coil and glass strand orbetween glass strand and crucible, which can lead to destruction of thecoil and/or of the crucible.

Furthermore, document FR-A 2 561 761 has disclosed an apparatus with aninductively heated, cold melting crucible for the ongoing removal ofmelted substances. The melt is removed via a discharge made fromrefractory material. Moreover, an inclined, pivotable retaining deviceis provided in the melting crucible, in front of the discharge.

Document FR-A 2 589 228 shows a similar apparatus for the continuousproduction of materials which are obtained from substances in the moltenstate. In this apparatus, the material overflows continuously from acold melting crucible, via a pipeline or channel, into a vessel.

However, the abovementioned apparatuses are in need of furtherimprovement, in particular with regard to the melting capacity and theglass quality which can be achieved.

It is an object of the invention to refine a melting method or apparatusof the known type in such a way that simple components which are asconventional as possible are used to link the melting unit to thefurther-processing stations and that the glass quality is not adverselyaffected by the forced use of a connecting technique in accordance withthe prior art.

This object is achieved by a method in accordance with claim 1 and anapparatus in accordance with claim 8.

Surprisingly, it has emerged that for uniform melting of the contents ofthe crucible it is not absolutely imperative for the entire volume ofthe melt to be arranged inside the induction coil, as is known from theprior art, a measure which in the prior art is supposed to result in thehigh-frequency energy being introduced as uniformly as possible in theentire volume of the melt but on the other hand means that it is notpossible for the melting crucible to be connected to the downstreamfurther-processing units using conventional components, on account ofthe disruption caused by the coil arrangement.

According to the invention, the glass level in the crucible projectsabove the upper end of the coil arrangement at least sufficiently farfor it to be possible for the glass outlet to be arranged completelyabove the coil arrangement. Furthermore, the outer end of the glassoutlet projects beyond the outer radius of the induction coil. Thismeans that the glass can be fed for further processing without flowingthrough the coil region. It is simple and inexpensive to connect themelting crucible to the next further-processing unit using conventionalcomponents, since there are no disruptive coils in the way. There are norestrictions whatsoever on the choice of subsequent components, andconsequently any possible adverse effects on the quality of the glassresulting from the type of connection can be minimized when a suitableconnection technique is selected.

It is surprising that the method and apparatus according to theinvention do not lead to any significant cooling of the melt above thecoil region. A major role in this context appears to be played by thetype of heating and the convective flows which are induced as a result.The high-frequency heating means that the hottest zone in the glass isgenerated in the middle of the melt volume, in the center of the coilgeometry. The crucible walls, by contrast, are cold, on account of thewater cooling. It appears that as a result a strong convectivecirculation is formed, transferring large quantities of heat out of thehot core into the upper, colder regions of the melt volume (cf. in thisrespect FIG. 1). The formation of the convective circulation can beenhanced further by additional bubbling, as will be explained in moredetail below.

It has been found that this effect can be boosted still further by theuse of the cooled bridge. Melt which passes into the region of thecooled bridge is cooled by the latter and drops downward toward thebase. A downward flow is formed, and this evidently generates a type of“cooling curtain” or “flow curtain” in the melt. This behavior likewiseboosts the circulation of the entire melt located in the melting regionof the crucible. The use of bridges in melting technology is inherentlyknown, and these bridges are usually employed in order to preventundissolved constituents of the batch from flowing through direct to theglass outlet. In addition to its purely mechanical separating actionbetween melting region and outlet region, the water-cooled bridgeaccording to the invention therefore also provides a thermal separationbetween the two regions by the formation of the above-described “coolingcurtain” or “flow curtain”. Consequently, the separating action of thebridge extends much further into the glass volume than its simplegeometric dimensions.

It is essential to the invention that this effect occurs even with onlyslight bridge penetration depths. In the method and apparatus accordingto the invention, excessively great penetration depths of the cooledbridge could lead to the glass outlet freezing up, since the upperregion of the melt volume and also the glass outlet are located abovethe region where the high-frequency energy is introduced.

The invention is explained in more detail below with reference to thefigures, in which:

FIG. 1 diagrammatically depicts a vertical section through a meltingcrucible in accordance with the invention (illustrating the convectioncirculations and the hot core zone);

FIGS. 2 a and b show two different embodiments of the baseplate of askull crucible;

FIGS. 3 a, b, c diagrammatically depict plan views of three preferredembodiments of the design and arrangement of the cooled bridge accordingto the invention;

FIG. 4 diagrammatically depicts a vertical section through a cooledbridge in a preferred embodiment;

FIG. 5 a, b diagrammatical depict a front and side view of thearrangement of the electrical short-circuiting connections in the outletregion;

FIG. 6 diagrammatically depicts a plan view from above of a meltingcrucible according to the invention with two outlets;

FIG. 7 a diagrammatically depicts a side view of a tank furnace with adirect connection between the HF melting crucible and platinum channelfor a continuous process;

FIG. 7 b diagrammatically depicts a side view of a tank furnace with afree-falling glass strand between the HF melting crucible and theplatinum channel.

FIG. 1 shows a melting apparatus in accordance with the invention,having a skull crucible 1. In the embodiment illustrated, the skullcrucible 1 comprises a cylindrical crucible wall 1.1.

Details of the crucible structure are described below but are not allillustrated in the figure, for the sake of clarity.

The crucible wall 1.1 is constructed from a ring of vertical metal tubeswhich are connected to one another at the top and bottom in meanderingform. The crucible base 1.2 may likewise comprise metal tubes orsegments, or may alternatively be made from refractory material.

The metal tubes are connected to at least one coolant feed and coolantdischarge. The coolant used is generally water. The coolant is guidedover a meandering path corresponding to the arrangement of the metaltubes. Depending on the size of the crucible, it is possible to providea plurality of coolant circuits for cooling individual ring segments. Inthe base region of the crucible, in the case of relatively small specialsteel crucibles with a volume of up to 50 l, the metal tubes are heldspaced apart from one another and are not connected to one another in anelectrically conductive manner. In the case of copper crucibles, it isalso possible to design larger crucibles of up to 100 l withspaced-apart tubes. In order in this case to prevent an electrical shortcircuit, by way of example mica platelets are positioned betweenadjacent tubes. In the case of crucibles with very large meltingvolumes, it may be expedient, as described in DE 199 39 780.5 A1, for anelectrical short circuit between the tubes also to be positioned in thebase region. At the upper end of the crucible, all the tubes areelectrically short-circuited with one another.

The base 1.2 of the skull crucible 1 is electrically insulated from thecrucible wall 1.1. This is achieved, for example, by means of a Quarzalor mica plate. Of course, it is also possible for other electricallynonconductive materials to be used here. The base 1.2 is likewise cooledand, as illustrated in FIG. 2, may, for example, comprise meanderingtubes 1.2.1 or metallic pieces 1.2.2 arranged in a similar manner toslices of cake. It is obvious that the invention is not restricted tothese specific configurations of the crucible. The invention alsoencompasses other crucible shapes, geometric arrangements of the metaltubes and configurations of the crucible base.

The crucible 1 is heated in the customary way by an induction coil 1.3which surrounds the crucible wall 1.1 and by means of whichhigh-frequency energy can be introduced into the contents of thecrucible. The glass melt 10 is located in the crucible 1. The surface ofthe glass melt 10 is denoted by 10.1 in FIG. 1.

The crucible has an inlet 1.4 for supplying batch and an outlet 1.5 fordischarging the molten glass.

It is essential to the invention that, as illustrated in FIG. 1, onlythe lower part of the crucible be surrounded by the induction coil 1.3.The crucible 1 is arranged in such a way relative to the coil 1.3 thatthe melt inside the crucible projects significantly out of the coilregion, i.e. the surface 10.1 of the melt 10 is located well above theupper end of the induction coil 1.3. The glass outlet 1.5 is likewisearranged above the upper end of the coil and extends beyond the outerradius of the induction coil. This ensures that the melt can be takenoff outside the region enclosed by the induction coil 1.3, i.e. withoutflowing through the coil region, and fed to the further-processingunits. Since the connecting location is outside the induction coilregion, the glass outlet 1.5 can be connected in a simple way to anyfurther-processing units by means of conventional components.

A further feature which is essential to the invention is the arrangementof a bridge 2 in the upper part of the crucible 1. The bridge 2 is atleast partially immersed in the melt and thereby separates the batchregion from the overflow and outlet region. Even slight immersion depthsare sufficient to form the flow curtain mentioned in the introduction.

The correct dimensioning of crucible, outlet and bridge and theirarrangement relative to one another depends on the individual case andcan easily be determined by the person skilled in the art at any time bymeans of a few simple routine experiments. The following values for thepositioning of the bridge, the coil and the glass outlet have provenexpedient for a skull crucible with a volume of 30 l: it has provenadvantageous for the bridge to be immersed in the glass melt to a depthwhich is such that its lower end is located approximately 1 to 2 cmbelow the bottom of the glass outlet 1.5. This in any event ensures thatundissolved constituents of the batch cannot flow through under thebridge and to the outlet. However, the bridge should advantageouslystill be located above the induction coil, preferably approximately 1 cmabove it. More preferably, the distance between the lower end of thebridge and the upper end of the coil should be 2 cm. In principle, it isalso possible for the end of the bridge to project into the coil region.In this case, however, it should be noted that the deeper the bridgepenetrates into the coil region, the more the high-frequency field isdisplaced, with the result that the extent to which it is introduceddrops and the melt in the outlet region may freeze.

The glass level in the outlet should be at least 2 cm. Otherwise, thereis again a risk of the melt freezing.

The outer end of the glass outlet should project at least 2 cm beyondthe outer circumference of the coil. If it projects by less than thisamount, it is necessary to provide electrical insulation for the systemswhich adjoin the glass outlet.

In general, the geometry of the bridge 2 can vary. FIG. 3 shows threedifferent preferred embodiments of a bridge. The figures each illustratea plan view of the cylindrical skull crucible 1 with glass outlet 1.5and a preferred variant of the bridge 2. The bridge 2 may have astraight cross section (FIG. 3 a), an angular cross section (FIG. 3 b)or a curved cross section (FIG. 3 c).

It can be seen from FIG. 3 a that with the “straight” embodiment of thebridge 2, what are known as “dead” zones 10.3, which are separated fromthe melting region by the bridge, are formed in the melt 10. In thesedead zones 10.3, the ratio of energy supplied by the high-frequencyradiation to energy dissipated by the cooling is very unfavorable, andthere is a risk of the melt freezing. Moreover, this area is notavailable to the melting region, which leads to losses of meltingcapacity. This effect is slight in the case of small crucibles, andtherefore in these crucibles the use of a straight bridge is stillrecommended for the sake of simplicity. Once larger melting volumesstart to be used (>70 l), it is expedient to take measures to returnthese zones to the melting region. Possible solutions are represented bythe round and angular bridges illustrated in FIGS. 3 b and c. In theseembodiments, the “dead” zones 10.3 are significantly reduced in size,with the angular variant offering the advantage of being simpler torealize in structural terms.

Irrespective of the specific embodiment, the bridge 2, like the entirecrucible wall 1.1, has to be composed of cooled, preferably metalliccomponents. In order to have the minimum possible effect on the highfrequency, the bridge 2 is preferably, as illustrated in FIG. 4,composed of individual tubes 2.2 which are arranged in meandering formwith respect to the coolant path and are connected to one another in anelectrically conductive manner at their upper ends. In this case, anelectrical short circuiting of both all the tubes 2.2 of the bridge 2 orbridge parts with respect to one another and with the skull crucible 1itself is also effected with the aid of an electrical contact (metalconnection).

In preferred embodiments of the invention, the melting crucible 1 andthe bridge 2 may be made from special steel, platinum, copper oraluminum. Which metal is expediently used depends on the composition ofthe glass to be melted and the demands imposed on its purity. If specialsteel or copper skull crucibles are used, the corrosion resistance ofthe crucible material can also be improved by application of a coatingof a highly thermally stable plastic, the decomposition temperature ofwhich is below the temperature of coating in the coating/melt contactregion. Examples of suitable plastics materials include plastics with ahigh fluorine content, in particular PTFE. A coating of this nature hasthe further advantage that exposed parts of the crucible are protectedfrom attack by components which evaporate out of the glass melt. Thethickness of the coating is to be such that the cooling by the metaltubes is still sufficient to keep the contact temperature betweencoating and melt below the decomposition temperature of the plastic.Coating with plastic has the further advantage that the glass does notstick to the coated parts and that the likelihood of sparkovers betweenthe metallic skull tubes is reduced. It is also possible for a cruciblemade from a metal with a relatively low resistance to corrosion to becoated with a metal with a higher resistance to corrosion.

In a preferred embodiment, the outlet 1.5 is formed by the cooled tubesof the skull crucible, which are bent over through 90° in the upperregion of the crucible. These tubes, as illustrated in FIG. 5, areconnected to one another in an electrically conductive manner at theirends, so that the outlet region also has a short-circuiting ring I,which surrounds the outlet 1.5 itself and the bridge 2 with it.Moreover, it has proven expedient to additionally provide a furthershort-circuiting section II, which connects all the tubes which havebeen bent over through 90° to one another in an electrically conductivemanner below the outlet 1.5 and in each case also surrounds the twotubes which delimit the outlet on both sides and lead upward again. Inthis way, the risk of HF sparkovers between the angled-off tubes of theoverflow region and the adjacent tubes of the skull crucible wall whichare not angled off is minimized.

In the case of crucibles in which a relatively great melting capacityfor a glass (>200 ml/min) is achieved, it is also possible to provide aplurality of (two to four) outlets. FIG. 6 shows an embodiment with twooutlets 1.5.1/1.5.2. In this exemplary embodiment, the outlets 1.5.1 and1.5.2 are positioned diametrically opposite one another (180° interval).This has the advantage that the homogenization units 6 can be arrangedas far away from one another as possible in spatial terms. By way ofexample, a plurality of outlets located closer together (e.g. at 90°intervals) are also possible. They could be arranged so close togetherthat they can be shielded from the batch by a common bridge. A cruciblewith a plurality of outlets has the advantage that if there is a needfor complicated hot-forming which can only process low throughputs ofglass, it is if desired possible to supply two lines with one crucible.This option is of particular interest if the glass requires no furtherrefining downstream of the melting section, since in this case all thatis required is second shaping, and there is no need for any Pt refiningchamber, which is expensive on account of the presence of Pt.

Good refining of the glass requires a relatively long residence time inthe refining unit, and consequently only relatively low glassthroughputs can be achieved. The maximum melting capacity of the HF unitcannot be utilized with one outlet and one refining chamber, since thereis a lack of refining chamber capacity. If two outlets and,correspondingly, two refining chambers are used, this drawback can becanceled out, but this solution is associated with the high Pt costs ofthe refining chamber. It would also be possible to carry out refiningusing a further HF unit rather than with a Pt refining chamber.

To increase the throughput or melting capacity, it is possible for themelting temperature to be increased virtually to any desired extent,since in this case there are no wall contact materials acting asrestricting parameters. Moreover, in particular in the case of glassmelts with a high viscosity, an agitation movement produced by bubblinghas a favorable effect on the melting capacity. Bubbling of this naturecan be carried out in a skull crucible (1) either by means of at leastone bubbling tube which is inserted from above, as illustrated in FIG.1, or by bubbling nozzles 1.6 positioned at the crucible base 1.2.

Further acceleration of the melting capacity can be achieved by the useof additional top heat in the region where the batch is laid. For thispurpose, it is possible to use either, as shown in FIG. 1, a burner 3 ordirect or indirect electrical heating.

If a burner is used to generate the top heat, it may be useful for theskull crucible to be designed as a mushroom skull, as described in DE199 39 772 C2, the content of disclosure of which is hereby incorporatedin its entirety. In this embodiment, the cooled metal tubes of the skullcrucible are bent off into the horizontal in the upper crucible regionbelow the melt surface, so that they form a cooled collar just below themelt surface. The temperature of the melt decreases toward the outsidein the region of the collar. The glass melt may in this case be cooledin the edge region of the collar to a sufficient extent for it to bepossible for a ring of refractory material to be fitted onto this edgeas a continuation of the crucible wall in the upper crucible region.With this arrangement, the cooled metal tubes are completely coveredwith glass melt on the side facing the melt and are therefore protectedfrom the corrosive action of the burner exhaust gases and theevaporation products from the melt. In return, the glass melt on thecooled tubes prevents the top space of the furnace from beingexcessively cooled by the crucible tubes.

In a further preferred embodiment of the invention, top heat isadditionally used in the overflow or outlet region in order to preventexcessive cooling of the melt in this region and to ensure the flow ofglass. For the configuration of the crucible 1 with a plurality ofoutlets 1.5, it is also advantageous for top heat to be introduced intoeach of the additional outlets, in order to allow a continuous flow ofgas out of all the outlets. As illustrated in FIG. 1, the top heat canbe generated by means of burners 4.

In a further preferred embodiment of the invention, a bridge 2 isconnected in front of each individual outlet 1.5. These bridges 2 areexpediently configured in such a way that the surface area of all the“dead” zones 10.3 together is kept as small as possible, in order toensure that the high frequency is introduced into the maximum possiblevolume of melt and therefore to ensure the highest possible melting ratefor the batch. However, embodiments in which a continuous bridge for aplurality of outlets is provided are also entirely conceivable.

The number of outlets which is most favorable for the specificsituation, the positioning of these outlets in the upper crucibleregion, the use of a common bridge or a plurality of bridges, and thegeometric configuration of the bridge(s) depend on the individualcircumstances and can easily be determined by the person skilled in theart without the need for any inventive step.

FIG. 7 a shows a melting crucible 1 according to the invention incombination with a platinum refining unit 5 and homogenization unit 6.An arrangement of this type can be used, for example, as a continuousmelting tank furnace for aggressive, high-purity glasses. In this case,there are various conceivable options for transferring glass betweenmelting crucible 1 and platinum refining chamber 5. For example, theplatinum refining chamber 5 may be flanged directly onto the metallicshort-circuiting ring at the outer end of the glass outlet 1.5 of themelting crucible 1. In a further preferred embodiment, the melt, asillustrated in FIG. 7 b, drops freely, in the form of a glass strand,into the platinum refining chamber 5 and there is no direct connectionbetween the melting crucible 1 and the downstream refining chamber 5 andthe homogenization apparatus 6. The drop height for the glass may inthis case be selected to be so small, since there are no coil turns inthe way, unlike in the known apparatus, that the introduction of bubblesand the formation of cords can be kept within tolerable limits.

Where extreme demands are imposed on purity and in particular on theabsence of platinum, it is recommended to use glass-meltinginstallations in which further tank furnaces, crucibles and/or channelswhich are heated by high frequency are used in addition to the meltingcrucible according to the invention, for example a high-frequencyrefining channel as described in DE 199 39 782 A1, DE 199 39 784 A1 orDE 199 39 786 A1, or a high-frequency refining crucible, as known fromDE 199 39 772 C1, all of which documents are hereby incorporated byreference in their entirety in the present disclosure.

If the demands imposed on the internal quality of the glass (bubbles,cords) are not particularly great—as is the case for example withsoldering glasses—the melting crucible according to the invention can beused to melt the glass without further additional components (refiningand homogenization crucible).

Exemplary Embodiment: Melting of High-Melting Aluminosilicate Glasses

An aluminosilicate glass (P1280) DE 19939771.6 having the compositionSiO₂=65.0% by weight; Al₂O₃=22.0% by weight; Li₂O=3.75% by weight;Na₂O=0.5% by weight; BaO=2.0% by weight; MgO=0.5% by weight; TiO₂=2.5%by weight; ZnO=1.75% by weight; ZrO₂=1.7% by weight and V₂O₅=0.3% byweight was melted in a melting crucible according to the invention, asillustrated in FIG. 1, made from Inconel 600®.

The effective melting volume in the crucible was approx. 25 l. A cooledbridge in accordance with the embodiment illustrated in FIG. 4 or FIG. 3a was used. The immersion depth of the bridge in the melt wasapproximately 50 mm for a glass level in the glass outlet of approx. 30mm, i.e. the bottom edge of the bridge was approx. 20 mm below thebottom of the glass outlet. This ensured that it was impossible for anyundissolved batch residues to flow through to the glass outlet under thebridge.

Melting crucible, bridge and glass outlet were electricallyshort-circuited with one another analogously to the embodimentillustrated in FIG. 5.

The glass was melted at an HF frequency of 386 kHz with generator powersof approx. 250 kW. In addition, the melting performance in the meltingregion was boosted by a burner and bubbling with oxygen. The meltedglass was taken off in accordance with the invention above the inductioncoil arrangement and fed for further processing without passing throughthe coil region. This was realized by virtue of the base of the glassoutlet lying 30 mm above the upper end of the induction coilarrangement. Furthermore, the outer end of the glass outlet projectedapprox. 70 mm beyond the outer coil radius.

Since the high viscosity of glasses of this type means that relativelylarge cross sections and throughputs or relatively high temperatureswere required in the overflow region, a burner was used in this regionto boost the outflow performance.

With the abovementioned melting volume of 25 l, the melting capacity ofthe melting unit was between 0.5 and 2 t of glass per day and wastherefore significantly higher than with the conventional meltingmethods mentioned in the introduction (1.0 t of glass per day for amelting volume of 90 l).

1. A method for the rapid melting of high-purity, aggressive and/orhigh-melting glasses in a skull crucible, comprising: introducinghigh-frequency energy into the glasses in the skull crucible by means ofa coil arrangement surrounding the skull crucible, in order to heat theglasses into molten glass; discharging the molten glass from an upperregion of the skull crucible; and retaining undissolved constituents ofthe molten glass by means of a cooled bridge which is immersed in themolten glass, wherein the upper region is above the coil arrangement anddoes not flow through the coil arrangement.
 2. The method as claimed inclaim 1, wherein the molten glass flows out of a glass outlet into acomponent of the next processing stage, which is fixedly connected tothe glass outlet, in a continuous process.
 3. The method as claimed inclaim 1, wherein the molten glass overflows at the glass outlet anddrops downward in the form of a glass strand.
 4. The method as claimedin claim 1, further comprising agitating the molten glass duringmelting.
 5. The method as claimed in claim 1, wherein the molten glassis taken off via a plurality of outlets.
 6. The method as claimed inclaim 2, further comprising adding heat to the upper region and/or theglass outlet to heat the molten glass.
 7. The method as claimed in claim1, further comprising refining and homogenizing the molten glass.
 8. Anapparatus for rapidly melting high-purity, aggressive and/orhigh-melting glasses, comprising: a skull crucible formed by a pluralityof cooled metal tubes arranged at intervals; a coil arrangementsurrounding the skull crucible for forming molten glass by introducinghigh-frequency energy into the contents of the skull crucible; a devicesfor discharging the molten glass, the device being arranged in an uppercrucible region; and a cooled bridge immersed in the molten glass, thecooled bridge separating the molten glass from the device fordischarging the molten glass, wherein the device for discharging themolten glass is a glass outlet arranged above the coil arrangement, theglass outlet having an outer end that projects beyond an outer radius ofthe coil arrangement, and wherein the cooled bridge comprises tubes thatare connected to one another in an electrically conductive manner. 9.The apparatus as claimed in claim 8, wherein the glass outlet is formedby the cooled metal tubes that are bent 90° outward in the region of theglass outlet.
 10. The apparatus as claimed in claim 9, wherein the outerend projects at least 2 cm beyond the outer radius of the coilarrangement.
 11. The apparatus as claimed in claim 8, wherein the glassoutlet has a base that is at least 2 cm above an upper end of the coilarrangement.
 12. The apparatus as claimed in claim 8, wherein the cooledbridge is immersed in the molten glass to a depth of at least 1 cm. 13.The apparatus as claimed in claim 8, wherein the cooled bridge has alower end that projects into the skull crucible between about 1 and 2 cmbelow a bottom of the glass outlet.
 14. The apparatus as claimed inclaim 8, wherein the cooled bridge comprises a plurality of cooled tubesarranged vertically next to one another and connected to one another attheir upper and lower ends.
 15. The apparatus as claimed in claim 8,wherein the cooled bridge, when seen in plan view from above, has ashape selected from the group consisting of rectilinear, round, andangular.
 16. The apparatus as claimed in claim 8, further comprising aplurality of glass outlets, wherein each glass outlet of the pluralityof glass outlets is assigned a different cooled bridge or a single,common cooled bridge.
 17. The apparatus as claimed in claim 8, whereinthe tubes of the cooled bridge are metal.
 18. The apparatus as claimedin claim 9, wherein the cooled metal tubes of the glass outlet have ashort-circuited ring at the outer ends.
 19. The apparatus as claimed inclaim 18, wherein the tubes of the cooled bridge are electricallyconductively connected to one another at their upper ends.
 20. Theapparatus as claimed in claim 17, further comprising an electricallyconductive connection between the glass outlet and cooled bridge. 21.The apparatus as claimed in claim 9, wherein the cooled metal tubes ofthe skull crucible, which form the glass outlet, below the bend areshort-circuited with one another and with in each case the first unbenttubes, adjacent to the glass outlet and leading upward, of the skullcrucible.
 22. The apparatus as claimed in claim 8, wherein the agitationdevice comprises at least one bubbling tube that is inserted into theskull crucible from above and/or arranged at a base of the skullcrucible.
 23. The apparatus as claimed in claim 8, further comprisingone or more devices for generating top heat in the skull crucible. 24.The apparatus as claimed in claim 23, wherein one or more devices aregas burners.
 25. The apparatus as claimed in claim 23, wherein the skullcrucible is designed as a mushroom crucible.
 26. The apparatus asclaimed in claim 8, further comprising a refining unit and ahomogenization unit connected downstream of the skull crucible.
 27. Themethod as claimed in claim 1, further comprising feeding the moltenglass directly to a shaping unit.
 28. The apparatus as claimed in claim8, further comprising a shaping unit connected downstream of the skullcrucible.